Wellbore survey systems used for geological surveying and drilling of oil and gas wells generally map or plot the path of a wellbore by determining wellbore azimuth and wellbore inclination at various points along the borehole. “Azimuth” may be considered, for present purposes, to be the directional heading relative to a reference coordinate, such as north. “Borehole inclination” may be considered, also for present purposes, the deviation from the vertical.
In WL logging operations, a logging system (tool) is conveyed into a wellbore after drilling. The logging tool mounts some sensors such as gyroscopes, accelerometers and magnetometers. The gyroscopes and the accelerometers are used for continuous measurements of attitude of the logging tool with respect to the gravity direction at each position in the wellbore. The magnetometer is used for to continuous measurements of azimuth at each position of the logging tool in the wellbore. By processing data from the sensors, the attitude of the logging tool with respect to the gravity direction and azimuth at each position of the logging tool conveyed in the wellbore are determined. The total traveling distance of the logging tool from the entrance of wellbore is determined by measuring cable length between the logging tool and the wellbore-entrance. The trajectory of the logging tool traveling in the wellbore can be determined based on the continuously measured data of the attitude and total traveling distance of the logging tool.
In drilling operations such as LWD and MWD operations, it is important to monitor azimuth and inclination of a wellbore and continually determine the position: and direction of the drilling tool to drill the wellbore as planned prior to drilling. The well bore azimuth and wellbore inclination can be monitored by measuring attitude and azimuth of the drilling tool. For this monitoring, the inclination and azimuth of the drilling tool located in the drilled wellbore are measured by utilizing the above-mentioned sensors such as gyroscopes, accelerometers and magnetometers mounted in the drilling tool.
Some navigation systems for wellbore survey using gyroscopes and accelerometers have been disclosed. For example, a strapdown inertial navigation system is disclosed in U.S. Pat. No. 6,453,239, issued Sep. 17, 2002, to Schlumberger Technology Corporation as the assignee of the inventors Ichiro Shirasaka, et al. This system employs three accelerometers mounted within a housing and three vibrating mass, Coriolis effect gyroscopes rigidly mounted within the housing.
In the wellbore survey using the gyroscopes and accelerometers, it is difficult to obtain stable and accurate output from the sensors under the ground because of harsh environment such as high temperature. Therefore, there is a need for sensor apparatuses to perform accurate measurements of azimuth and inclination for wellbore survey using gyroscopes and accelerometers even if such sensors are used, for example, in oilfield and any other harsh environment.
As will become apparent from the following description and discussion, the present invention provides an improved sensor apparatus capable of operating stably and accurately in harsh environment such as high temperature.